1. Field of the Invention
This invention is a copolymer composition that has an unusual and superior hydrolytic and thermal stability for use in a geothermal or other environment with high temperature and steam. The high temperature is often accompanied by brine or saturated salt solution of magnesium, sodium, calcium, bromine and other elements which degrade rubber products under these conditions. The composition contains carbon to carbon crosslinkage between the copolymer chains generated by a peroxide curing agent. The copolymer has superior crosslinkage efficiency which gives a higher tensile strength and modulus. The composition cam be made with a viscosity usable for transfer and compression molding of products. The products of the composition, such as pipe protectors and valve seals, have a longer life under the extreme conditions of geothermal service.
2. Description of the Prior Art
There has been a need for rubber or polymer formulations for use in geothermal or high temperature and steam applications. Natural and synthetic rubber have been stabilized by sulfur crosslinkage vulcanization. The polymer chains are crosslinked by one or more sulfur atoms bridging the chains. Under extreme heat and steam encountered in a geothermal system, sulfur vulcanized rubber tends to break down. The rubber becomes inelastic, brittle and does not perform adequately. The life of rubber parts can be only a few days in equipment for geothermal wells given the temperature conditions rising to 600.degree. F. with steam. With such a short effective life there is obviously high expense in replacing the rubber parts as well as down time and field personnel costs. A more stable rubber product has been sought to lengthen replacement time for rubber parts, as well as a rubber which has the resilience, tensile strength and resistance to abrasion for use in the geothermal type environment.
The need for a more stable rubber or polymer under high temperature and steam led to a search for a more stable rubber composition. The U.S. Department of Energy, as part of its project to develop alternative energy sources, explored the use of geothermal wells. During this investigation the need for an improved rubber material for seals and other rubber parts for geothermal applications was recognized. The Department of Energy contracted with L'Garde, Inc. of Newport Beach to develop a formulation which was designated L'Garde Compound Y267. A typical formulation of the L'Garde composition includes a terpolymer of ethylene-propylene-hexadiene marketed under the trademark Nordel 1660 by DuPont Elastomer Division of Wilmington, Delaware and a coagent polymer of 1-2 polybutadiene at 20 parts per 100 parts of the Nordel 1660 terpolymer. A dicumyl peroxide such as Dicup R, a trademark of Hercules, Inc. of Wilmington, Del., in 3.5 parts per 100 parts of rubber is used to effect carbon-carbon crosslinkage. However, the use of the terpolymer and polybutadiene produces a network of crosslinkage that includes a bond with a branch group on the polymer chain or linkage with the coagent which is not part of a uniform polymer chain. Also, the terpolymer used has a high diene content which promotes a faster cure and this usage is contrary to the accepted standards that a lower diene ages better. This type of crosslinkage is not as stable as a uniform crosslinkage between the backbones of the polymer chains without branch groups.
The L'Garde formulation also includes a halogenated hydrocarbon, chlorosulfonated polyethylene, under the trade mark Hypalon 20, a DuPont product at 5 parts per hundred of the terpolymer and antimony oxide Thermoguard S a product of M&T Chemicals, Rahway, N.J., at 5 parts per hundred of terpolymer. Antioxidant 2246 (2,2'-methylene-bis(4-methyl-6-t-butyl phenol) an American Cyanamide product is used in a very low amount of 0.50 parts per hundred of terpolymer. The carbon black is an ASTM N100 type at 75 parts per hundred of the terpolymer.
The L'Garde composition has other drawbacks in addition to the lack of uniform crosslinking between carbon chains. The halogenated hydrocarbon is not recommended for usage in steam service much less geothermal service. The compound increases water absorption and may cause blistering in steam. The L'Garde formula contains a very low level of anitoxidant which effectiveness is relative to the level in the rubber. The coagent, 1-2butadiene, used by L'Garde causes the rubber to be hard and inelastic. This quality narrows the uses of the composition which require elasticity such as pipe protectors.
An improvement of the L'Garde Y267 was developed by Parker Seals of Culver City, Calif., Product No. E962-85. Although there is test data available, the compound formula E962-85 is not disclosed. From the test results Parker Seals finds the E962-85 superior to L'Garde Y267. However, after 168 hours in steam at 600.degree. F., the compound lost 78% of its tensile strength.
The areas of use of an ethylene and propylene copolymer which cannot be sulfur crosslinked are electrical insulation, automotive applications, plastics modification, and electrical cable and connectors. Vistalon 404 is an Exxon Chemical America's trademark for a copolymer which is a 40.+-.5% ethylene and propylene 60.+-.5% product. A typical Exxon formulation using Vistalon 404 for use in the electrical insulation area includes peroxide coagent cure of 40% dicumyl peroxide and coagent TAC-75, a trade name product of Wyrough & Loser, which is a 75% dispersion of triallyl cyanurate. The formulation of the Exxon example have high water absorbency. Also, the composition favors conductivity while sacrificing processability and strength properties for producing products for use in geothermal service. The amount of antioxidant used is not large enough for geothermal protection and the TAC-75 coagent is not suitable for molds because it can cause sticking and premature vulcanization. The uses of the ethylene propylene copolymer for the drilling applications has not been identified.